At present, EAP (Electro Active Polymers) based actuators and generators (i.e. EAP based energy converters) are operated by either passive harvesting systems, where the deformation itself of an EAP based deformable body which acts as a variable capacitor, forces a flow of energy, or by active systems that control the electrical field, voltage or charge directly. As the latter approach typically yields higher conversion efficiencies and higher energy densities, it is preferred for especially larger systems or efficiency sensitive (battery-powered) applications.
An active system is for example disclosed in WO 2010/146457.
The way the electric field is established and controlled during the excitation determines the amount of energy that is converted. This is described by energy harvesting cycles; at present mainly three distinctive cycles are used in systems that charge and discharge the EAP actively; constant-charge, constant-voltage and constant-field cycles (SRI International). The focus of these cycles is on the way the power electronic unit (PEU) interacts with the EAP device during stretching or contraction (or relaxation). In these periods, most of the electromechanical conversion takes place.
Although Electro Active Materials are well known for their capability of handling large mechanical deformation (up to 500%), in many practical applications the deformation is limited; not only due to the nature of the application (such as excitation by waves) but also to limit the effect of fatigue.
Since the electromechanical transduction is based on interaction with an electric field, in applications with small deformation levels, the amount of energy required to cyclically bias the EAP device with an operating electric field is much larger than the amount of energy that is actually available for conversion. This increases the required power rating of the Power Electronic Unit, challenges the electromechanical conversion efficiency and adds cost and volume to an EAP based energy conversion system.
In electromechanical conversion applications with distributed sources, such as wave energy converters (as disclosed in WO2010011562) or rotational-to-reciprocating energy converters (as disclosed in WO2013059562), where an essential phase-shift exists between the different EAP devices within the application the required bias energy can be exchanged internally between the different EAP devices. In such multi-phase systems, the inherent ability of EAP devices to store energy is used effectively, offering distinctive advantages with respect to power capacity requirements, converter efficiency constraints and power quality.
Single-phase EAP based energy conversion systems such as described in PCT/EP2013/059614 have a dedicated Power Electronic Unit (PEU) or converter for each EAP device. This provides full controllability on the harvesting cycle applied to each EAP device, but since the bias energy needs to be applied cyclically by the PEU, it suffers from high-to-average power ratings, which in turn results in high converter cost, low electromechanical conversion efficiency with a very strong sensitivity to the PEU efficiency and as a result thereof, a need for technologically advanced converter implementations.
To limit the effect of the high peak to average power rating, some of the existing single-phase systems already employ some kind of harvesting strategy that attempts to optimize the energy conversion cycle such as published by Graf and Maas, “Optimized Energy Harvesting based on Electro Active Polymers”, 2010 International Conference on Solid Dielectrics, Potsdam, Germany, Jul. 4-9, 2010, by Graf, Maas and Schapeler, “Optimized Energy Harvesting based on Electro Active Polymers”, 2010 International Conference on Solid Dielectrics, Potsdam, Germany, Jul. 4-9, 2010, and by R. van Kessel, B. Czech, P. Bauer, and J. Ferreira, “Optimizing the dielectric elastomer energy harvesting cycles,” IECON 2010, 36th Annual Conference on IEEE Industrial Electronics Society 2010, pp. 1281-1286 (http://dx.doi.org/10.1109/IECON.2010.5675554)
However, these strategies focus on the EAP-to-PEU conversion stage only and not the overall system output power quality, and do not fully overcome the inherent deficiencies of single-phase energy conversion and the associated high peak to average power rating.
On the other hand, the multi-phase EAP energy conversion systems to date, such as described in WO 2010/146457 and WO 2011/044901, mostly use passive components in order to lower the amount of power that needs active processing. Whereas these systems are normally fairly effective in lowering the required active PEU power rating and also in providing some kind of system output power smoothening, individual control of the EAP sections in the system is barely possible due to the limited number of control inputs. Individual control of EAP sections is a prerequisite for operating at high electric field strengths and hence, high energy output, especially when irregular excitation sources are considered.
It is therefore an object of the invention to provide a system and method that overcome or mitigate the disadvantages of the prior art.